The objective of this grant application is to examine the mechanism of action of a novel phosphaturic factor, secreted frizzled-related protein-4 (sFRP-4), one of the factors responsible for the phosphaturia, hypophosphatemia and inappropriately adapted 25-hydroxyvitamin D3 1 alpha-hydroxylase (25(OH) D3 1alphaOHase) activity seen in patients with tumor-induced osteomalacia, and to determine whether it regulates phosphate (Pi) homeostasis in normal physiological states. The hypotheses of this application are as follows: (1) sFRP-4 regulates inorganic phosphate (Pi) homeostasis by two distinct mechanisms: (a) inhibiting renal proximal tubule Pi reabsorption; (b) inhibiting renal 25(OH) D3 1alpha (OH)ase activity and 1alpha, 25-dihydroxyvitamin D synthesis. These actions reduce intestinal Pi absorption and renal Pi reabsorption and are likely modulated by the effects of sFRP-4 on the Wnt signaling pathway. (2) Renal and circulating sFRP-4 is regulated by dietary Pi intake. The specific aims of the application are as follows: (1) To determine the site and mechanism of action of sFRP-4 in rat or mouse nephron. (2) To determine the structural basis for the phosphaturic effects of the sFRP-4 molecule. (3) To examine the physiological mechanisms by which sFRP-4 inhibits 25(OH)D3 1alpha (OH)ase activity in the proximal tubular cell. (4) To examine the signaling pathways activated in the proximal tubule by sFRP-4. (5) To determine whether sFRP-4 concentrations in the serum and kidney are altered by changes in the intake of dietary Pi. Significance: While the vitamin D endocrine system and PTH have been described as modulators of calcium and Pi homeostasis, no molecules or systems have been described that predominantly regulate Pi homeostasis, sFRP-4 may be such a molecule. We believe that sFRP-4 is an important molecule in this regard, and it is therefore critical to understand how it functions and is regulated in the kidney. Our findings will have important implications with respect to Pi homeostasis, bone mineralization in mammals and humans, and the regulation of ion transport through Wnt signaling in the kidney.